シャルピー式衝撃曲げ試験における歪分布について

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  • On the Strain Distributions of Broken Charpy Test Pieces
  • シャルピーシキ ショウゲキ マゲ シケン ニ オケル ワイ ブンプ ニ ツイテ

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The object of this paper is to observe the strain distributions of Charpy V-notch test pieces broken in the range of 100°C to −196°C. Some fine-grained (grain size 10μ) and coarse-grained (grain size 78μ) test pieces of a low carbon steel were tested. The transition temperatures were evaluated at −10°C for the fine grained samples, and at 40°C (Trf) and at 0°C (Trd) for the coarse grained samples based on the load-time curves and the absorbed energy-temperature curves. The strain distributions were examined on the longitudinal outer surface (named X·Y plane), and the central section (named (X·Y)M plane) which are both vertical to the notch of test piece. The strain distributions at the surface and the central section were measured by means of the microhardness test and the recrystallized grain observation, and also the amount of strain on the surface was confirmed by the distortion of the fine intercrossing which were printed photographically. The results obtained in this study are summarized as follows: (1) The strained region of (X·Y)M plane spreads less than that of (X·Y) plane in all the cases. (2) Above the transition temperature, a strain of more than 10% appears over a wide region of 4∼8 mm in breadth along the notch and the fractured face on the (X·Y) plane, and of 4∼5 mm in breadth along the fractured face only on the (X·Y)M plane. The minimum amount of strain at about 1 mm apart from the fractured face is estimated to be 30% on the (X·Y) plane and 25% on the (X·Y)M plane. (3) At the transition temperature, the strain of about 10% appears over a region of 1.5∼2 mm in breadth nearly parallel to the fractured face on the (X·Y) plane. The strained region on the (X·Y)M plane, however, spreads circularly in a narrow band below the notch, and the amount of strain in this band is estimated to be about 7∼8%. (4) Below the transition temperature, a little strained region appears in the vicinity of the fractured face on the (X·Y) plane: besides this, strained regions are hardly detectable except some twins on (X·Y)M plane. (5) The volume of strained region corresponds to the crack initiation energy which is estimated from the load-time curve. (6) Although the transition temperature depends distinctly on the grain size of the test piece, the size of strained region is mainly affected by the test temperature level, not by the grain size.

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